The present invention relates generally to the field of nonaqueous electrolytes and electric current producing cells. More particularly, this invention pertains to non-aqueous electrolytes which comprise (a) one or more solvents; (b) one or more ionic salts; and, (c) a multifunctional monomer. When incorporated into a nonaqueous electrolyte, the multifunctional reactive monomer improves the safety of electric current producing cells by rapidly polymerizing at elevated temperatures to increase the viscosity and internal resistivity of the electrolyte. The present invention also pertains to electric current producing cells comprising such non-aqueous electrolytes, methods of making such non-aqueous electrolytes and electric current producing cells, and methods for increasing the safety of an electric current producing cell.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
Electric current producing cells, and batteries containing such cells, consist of pairs of electrodes of opposite polarity separated by an electrolyte. The charge flow between electrodes is maintained by an ionically conducting electrolyte.
Successful use of batteries depends on their safety during operation under normal conditions and even under abusive usage. An abusive use such as short circuiting or rapid overcharging of the battery may initiate self-heating of the battery, as opposed to merely resistive heating, leading to thermal runaway. Self-heating of the battery is especially problematic when a lithium anode is utilized. Lithium batteries are capable of much higher power storage densities than batteries not based on lithium, but the reactivity of lithium to most materials and its melting point of only about 180xc2x0 C. promote self-heating and the potential for thermal runaway. This reactivity of lithium to the electrolyte and other materials of the battery is typically increased in secondary or rechargeable lithium cells upon the cycling of the lithium. The main processes causing self-heating of a secondary lithium cell typically involve the chemical reaction between cycled lithium and electrolyte solvent. These self-heating reactions are believed to be initiated at temperatures near 100xc2x0 C. At temperatures greater than 100xc2x0 C., additional contributions to cell self-heating are believed to come from exothermic decomposition of the electrolyte as well as from reactions between lithium and the ionic salt in the electrolyte.
One approach to reduce the potential of unsafe explosive conditions with lithium batteries has been to use blends of electrolyte solvents which have a lower reactivity with lithium. For example, U.S. Pat. No. 4,753,859 to Brand et al. describes the use of one or more polyethylene glycol diallyl ethers with ethylene carbonate and propylene carbonate as the electrolyte solvents to improve the safety characteristics of a nonaqueous lithium cell. Also, for example, U.S. Pat. No. 5,219,684 to Wilkinson et al. describes the use of sulfolane and a glyme as the electrolyte solvents for improved safety in electrochemical cells with a lithium-containing anode and a cathode with a lithiated manganese dioxide as the active material. Although it is beneficial to safety to select the electrolyte solvent composition to reduce the reactivity with the lithium, solvent choice alone does not provide strong protection against overheating and thermal runaway. Among the disadvantages of this approach of using less reactive electrolyte solvents are that the battery is not shutdown or reduced in current flow at high temperatures, but is still capable of high energy electrochemical reactions leading to more heat buildup and degradation reactions which may eventually result in an explosive condition. Also, the electrolyte solvent composition may not be compatible with the specific cathode composition used in the cell. During the discharging and charging of electrochemical cells, many electrochemically reduced and oxidized compounds are formed which may not be stable or otherwise compatible with the electrolyte solvents. For example, solid sulfur-based cathodes are very desirable for use with lithium-containing anodes because of the extremely high power density of this combination of electroactive materials, but the organic and inorganic polysulfides typically formed in the discharge or reduction cycle of these nonaqueous lithium-sulfur type batteries may not be compatible with one or more solvents, such as ethylene carbonate, in the electrolyte solvent blend selected for safety.
Another approach to improve the safety of electrochemical cells has been to incorporate a PTC (positive temperature coefficient) device that has increased electrical resistance at high temperatures and thereby suppresses the flow of current through the cell as, for example, described in U.S. Pat. No. 4,971,867 to Watanabe et al. and U.S. Pat. No. 5,008,161 to Johnston. PTC devices may provide useful safety protection against internal short circuits and against overcharge and overdischarge conditions, but they are not adequate to substantially shutdown or reduce the activity of the reactive chemistry in a cell.
To control overheating under abusive usage, it has been suggested that a thermally activated separator be developed for insertion between the cathode and the anode. It has been further suggested that a microporous sheet might function as this thermally activated separator if it exhibited low resistivity at normal operating temperatures but irreversibly transformed into a product having high resistivity at high temperatures, while maintaining its dimensional integrity. For example, U.S. Pat. No. 4,650,730 to Lundquist et al. describes a multi-ply microporous sheet useful as a battery separator having at least two plies with different transformation temperatures for forming a substantially non-porous sheet at elevated temperatures. Microporous polymeric films presently employed as separators in lithium batteries are generally not capable of preventing uncontrolled heating. In general, polymeric separators degrade, to one extent or another, under the influence of heat and thermal reactions, or become dimensionally unstable, and they do not substantially reduce or shutdown the activity of the reactive chemistry of a cell.
It has been suggested by Laman et al., J. Electrochem. Soc., 1993, 140, L51 to L53, that for a separator to function well as an internal safety device in a lithium battery, it should have the following characteristics: a melting point close to 100xc2x0 C. for the low melting point component, a high dimensional stability temperature preferably above the melting point of lithium, and a high degree and rate of shutdown, giving rise to an impedance increase of at least three orders of magnitude with an increase of a few degrees Celsius in temperature. They note that it is difficult to achieve these properties in a single separator and that obtaining all these characteristics can be more easily achieved by combining different separators. Using a combination of different polymeric separators, especially when the surface area of separators required in the battery is very large, significantly increases the expense of producing the battery, as well as reducing the volume available for electroactive material, thereby reducing the specific capacity of the cell.
To overcome the safety disadvantages of conventional polymeric films as battery separators, one approach has been to add a thermal fuse material which melts at high temperatures to the polymeric film so that the thermal fuse material melts and irreversibly reduces the porosity of the microporous polymeric films, thereby interrupting the electrochemical reaction in the, battery. For example, U.S. Pat. No. 4,973,532 to Taskier et al. describes a battery separator with a thermal fuse material adhered to a porous substrate in a predetermined geometric array.
As an alternative to a melting transformation of the battery separator material to provide safety protection, a temperature-induced change in the electrolyte composition has been suggested. For example, U.S. Pat. No. 5,506,068 to Dan et al. describes a liquid electrolyte solution containing a solvent which rapidly polymerizes at a temperature exceeding 100xc2x0 C. to cause an increase in the internal resistivity of the cell and to safely terminate the operation of the cell. They noted that this advantageous result had only been observed with 1,3-dioxolane as the solvent in the presence of a manganese dioxide based cathode and certain lithium based ionic salts and suggested that it was conceivable that other combinations of electrode, solvent, and salt may produce a similar result. U.S. Pat. No. 4,952,330 to Leger et al. describes an organic electrolyte containing 40 to 53 volume per cent of a polymerizable component of a cyclic ether, preferably 1,3-dioxolane is the cyclic ether, that will effectively prevent internal shorting of cells when subjected to certain kinds of abusive conditions.
Besides temperature-induced effects by the solvent of the electrolyte to provide safety protection, electrolytes containing additives which polymerize or melt to increase the internal resistance of the battery have been utilized for safety protection. For example, Eur. Pat. Application No. 759,641 to Mao describes the addition of a monomer additive, such as biphenyl or other aromatic additives, to the liquid electrolyte of a lithium anode based battery with a lithium insertion compound cathode to protect the battery during overcharge by polymerization of the additive at battery voltages greater than the maximum operating voltage. Also, for example, U.S. Pat. No. 5,534,365 to Gee et al. describes the use of dispersed particles of an inert fusible material in a solid polymer electrolyte to melt at temperatures in the range of 80xc2x0 C. to 120xc2x0 C. to provide safety by increasing the impedance of the solid electrolyte.
Another temperature-induced approach to safety protection in batteries is to release a poisoning agent, which deactivates the battery, when a certain temperature is reached. For example, U.S. Pat. No. 4,075,400 to Fritts describes an encapsulated battery poisoning agent where the encapsulant melts at a determined melting point to provide overheating and overload protection to the battery. U.S. Pat, No. 5,714,277 to Kawakami describes electrolyte solutions or separators comprising microcapsules that melt in the temperature range of 70-150xc2x0 C. and contain a chemical substance which is discharged when the microcapsules melt. The chemical substance may be a polymerization initiator, a compound having a hydroxyl group, an acid, a cross-linking agent, or a flame retardant. The microcapsules may optionally contain monomer, oligomer, or polymer. U.S. Pat. No. 5,580,680 to Chaloner-Gill et al. describes a solid electrolyte that includes one or more catalysts that are capable of initiating the polymerization of the solvent component of the electrolyte at elevated temperatures. Microcapsules may be used that permit the controlled release of the catalysts into the electrolyte under the appropriate conditions.
It would be advantageous to the art of battery design, particularly for secondary lithium batteries, if the electrolyte would function as the electrolyte up to a specified initiation temperature, and thereafter, the electrolyte underwent a rapid and irreversible transformation, such as an increase in viscosity and internal resistivity, to reduce the ionic conductivity of the electrolyte, thereby providing increased safety to the battery, such as, for example, safely preventing overheating and thermal runaway, and for example, safely terminating the operation of the battery.
One aspect of the present invention pertains to a nonaqueous electrolyte for use in an electric current producing cell, the electrolyte having a viscosity and an internal resistivity, the electrolyte comprising (a) one or more solvents; (b) one or more ionic salts; and, (c) a multifunctional monomer comprising two or more unsaturated aliphatic reactive moieties per molecule, which multifunctional monomer is soluble in said one or more solvents, which multifunctional monomer rapidly polymerizes when said electrolyte is heated to an initiation temperature greater than 100xc2x0 C., thereby increasing said viscosity and internal resistivity of said electrolyte. This polymerization increases the viscosity and internal resistivity of the electrolyte and increases the safety of the cell. In one embodiment, upon rapid polymerization of the electrolyte at the initiation temperature, the viscosity and internal resistivity of the electrolyte increase to a level sufficient to safely terminate current producing operation of the cell.
In one embodiment, the reactive moieties of the multifunctional monomer of the nonaqueous electrolyte of the present invention are selected from the group consisting of: vinyl (CH2xe2x95x90CHxe2x80x94), allyl (CH2xe2x95x90CHxe2x80x94CH2xe2x80x94), vinyl ether (CH2xe2x95x90CHxe2x80x94Oxe2x80x94), allyl ether (CH2xe2x95x90CHxe2x80x94CH2xe2x80x94Oxe2x80x94), allyl ester (CH2xe2x95x90CHxe2x80x94CH2xe2x80x94C(O)xe2x80x94Oxe2x80x94), allyl amine (CH2xe2x95x90CHxe2x80x94CH2xe2x80x94NHxe2x80x94), acrylyl (CH2xe2x95x90CHxe2x80x94C(O)xe2x80x94), acrylate (CH2xe2x95x90CHxe2x80x94C(O)xe2x80x94Oxe2x80x94), acrylainde (CH2xe2x95x90CHxe2x80x94C(O)xe2x80x94NHxe2x80x94), methacrylyl (CH2xe2x95x90C(CH3)xe2x80x94C(O)xe2x80x94), methacrylate (CH2xe2x95x90C(CH3)xe2x80x94C(O)xe2x80x94Oxe2x80x94), and crotoxyl (CH3xe2x80x94CHxe2x95x90C(CH3)xe2x80x94C(O)xe2x80x94).
In one embodiment, the multifunctional monomer is a multifuictional vinyl ether monomer, preferably a vinyl ether monomer selected from the group consisting of; divinyl ether monomers, trivinyl ether monomers, and tetravinyl ether monomers. In a more preferred embodiment, the multifunctional monomers is a divinyl ether monomer. In a most preferred embodiment, the divinyl ether monomer is selected from the group consisting of: divinyl ether of ethylene glycol, divinyl ether of diethylene glycol, divinyl ether of triethylene glycol, divinyl ether of tetraethylene glycol, divinyl ether of 1,4-butanediol, and divinyl ether of 1,4-cyclohexanedimethanol.
In one embodiment, the nonaqueous electrolyte of the present invention is selected from the group consisting of: liquid electrolytes, gel polymer electrolytes, and solid polymer electrolytes. In a preferred embodiment, the electrolyte is a liquid electrolyte.
In one embodiment, the one or more solvents of the nonaqueous electrolyte of this invention comprise a solvent selected from the group consisting of: N-methyl acetamide, acetonitrile, organic carbonates, sulfolanes, sulfones, N-alkyl pyrrolidones, dioxolanes, glymes, siloxanes and xylenes.
In one embodiment, the one or more ionic salts of the nonaqueous electrolyte of this invention comprise a lithium salt.
In one embodiment, the weight ratio of the combined total weight of multifunctional monomers present to the combined total weight of the one or more solvents in the nonaqueous electrolyte of this invention is in the range of 0.01:1 to 0.25:1. In a preferred embodiment, the weight ratio is in the range of 0.02:1 to 0.15:1 and, more preferably, the weight ratio is in the range of 0.02:1 to 0.1:1.
In one embodiment, the nonaqueous electrolyte of the present invention further comprises a monofunctional monomer having one unsaturated aliphatic reactive moiety per molecule, which monofunctional monomer is soluble in the one or more solvents of the electrolyte, which monofunctional monomer rapidly reacts with the multifunctional monomer when the electrolyte is heated to an initiation temperature greater than 100xc2x0 C. In at preferred embodiment, the monofunctional monomer of the present invention is selected from the group consisting of: vinyl ethers and allyl ethers. In a more preferred embodiment, the monofunctional monomer is a vinyl ether selected from the group consisting of: vinyl ether of 1-butanol, vinyl ether of 2-ethylhexanol, and vinyl ether of cyclohexanol.
In one embodiment, the weight ratio of the combined total weight of the monofunctional monomers present to the combined total weight of the one or more solvents and the multifunctional monomers present is in the range of 0.005:1 to 0.2:1. In a preferred embodiment, the weight ratio is in the range of 0.01:1 to 0.1:1.
In one embodiment, the nonaqueous electrolyte of this invention further comprises a polymerization initiator to increase the rate of polymerization of the multifunctional monomer. In a preferred embodiment, the polymerization initiator comprises a cationic polymerization initiator. In a more preferred embodiment, the polymerization initiator comprises a lithium ion.
In one embodiment, the nonaqueous electrolyte of the present invention further comprises an polymerization inhibitor to decrease the rate of polymerization of the multifunctional monomer. In a preferred embodiment, the polymerization inhibitor is a tertiary amine.
In one embodiment, the nonaqueous electrolyte of this invention further comprises a solid porous separator. In one embodiment, the solid porous separator is a porous polyolefin separator. In one embodiment, the solid porous separator comprises a microporous pseudo-boehmite layer. In one embodiment, the solid porous separator of the nonaqueous electrolyte does not melt at a temperature which is greater than 100xc2x0 C. and which is the lowest temperature at which polymerization of the multifunctional monomer rapidly occurs to increase the viscosity and internal resistivity of the electrolyte.
In one embodiment, the multifunctional monomer in the nonaqueous electrolyte of the cell does not polymerize if kept at a temperature of 60xc2x0 C. for 72 hours. In one embodiment, the multifunctional monomer in the nonaqueous electrolyte of the cell does not polymerize if kept at a temperature of 80xc2x0 C. for 72 hours.
In one embodiment, the multifunctional monomer in the nonaqueous electrolyte of the cell of the present invention does not rapidly polymerize at temperatures less than 100xc2x0 C. In one embodiment, the multifunctional monomer in the nonaqueous electrolyte of the cell of the present invention does not rapidly polymerize at temperatures less than 110xc2x0 C. In one embodiment, the multifuictional monomer in the nonaqueous electrolyte of the cell of the present invention does not rapidly polymerize at temperatures less than 120xc2x0 C. In one embodiment, the multifuictional monomer in the nonaqueous electrolyte of the cell of the present invention does not rapidly polymerize at temperatures less than 130xc2x0 C. In one embodiment, the multifunctional monomer in the nonaqueous electrolyte of the cell of the present invention does not rapidly polymerize at temperatures less than 150xc2x0 C. In one 1 embodiment, the multifunctional monomer in the nonaqueous electrolyte of the cell of the present invention does not rapidly polymerize at temperatures less than 170xc2x0 C.
Another aspect of the present invention pertains to a method of preparing a nonaqueous electrolyte for use in an electric current producing cell, the electrolyte having a viscosity and an internal resistivity, the method comprising the steps of (a) preparing a solution of one or more solvents; one or more ionic salts; and a multifunctional monomer comprising two or more unsaturated aliphatic reactive moieties per molecule, which multiflntional monomer is soluble in said one or more solvents, which multifunctional monomer rapidly polymerizes when the electrolyte is heated to an initiation temperature greater than 100xc2x0 C., thereby increasing the viscosity and internal resistivity of the electrolyte; and, (b) optionally combining said solution with other materials selected from the group consisting of: solid porous separators, ionically conductive polymers, and monoftuctional monomers having one unsaturated aliphatic reactive moiety per molecule. The multifunctional monomer rapidly polymerizes when the electrolyte is heated to an initiation temperature greater than 100xc2x0 C., thereby increasing the viscosity and internal resistivity of the electrolyte and increasing the safety of the cell.
Still another aspect of the present invention pertains to an electric current producing cell comprising (i) a cathode, (ii) an anode, and (iii) a nonaqueous electrolyte interposed between said cathode and said anode, said electrolyte having a viscosity and an internal resistivity, said electrolyte comprising: (a) one or more solvents; (b) one or more ionic salts; and, (c) a multifunctional monomer comprising two or more unsaturated aliphatic reactive moieties per molecule, which multifuntional monomer is soluble in the one or more solvents, which multifunctional monomer rapidly polymerizes when the electrolyte is heated to an initiation temperature greater than 100xc2x0 C., thereby increasing the viscosity and internal resistivity of the electrolyte. Upon the rapid polymerization of the multifunctional monomer, the viscosity and internal resistivity of the nonaqueous electrolyte is increased, thereby increasing the safety of the cell. In one embodiment, upon rapid polymerization of the electrolyte at the initiation temperature, the viscosity and internal resistivity of the electrolyte increase to a level sufficient to safely terminate current producing operation of the cell.
In one embodiment, the electric current producing cell is a secondary battery.
In one embodiment, the cathode of the cell of the present invention comprises an electroactive transition metal chalcogenide. In another embodiment, the cathode comprises an electroactive conductive polymer. In another embodiment, the cathode comprises an electroactive sulfur-containing material. In one embodiment, the electroactive sulfur-containing material of the cathode comprises elemental sulfur. In one embodiment, the cathode comprises an electroactive sulfur-containing material which, in its oxidized state, comprises a disulfide group.
In one embodiment, the cathode of the cell of the present invention comprises an electroactive sulfur-containing material, wherein the electroactive sulfur-containing cathode material, in its oxidized state, comprises a polysulfide moiety of the formula, xe2x80x94Smxe2x80x94, wherein m is an integer equal to or greater than 3, preferably m is an integer from 3 to 10, and more preferably m is an integer from 6 to 10. In a most preferred embodiment, the electroactive sulfur-containing cathode material is a carbon-sulfur polymer. In one embodiment, the electroactive sulfur-containing cathode material comprises a carbon-sulfur polymer selected from the group consisting of: carbon-sulfur polymer materials with their xe2x80x94Smxe2x80x94 groups, as described above, covalently bonded by one or more of their terminal sulfur atoms on a side group on the polymer backbone chain; carbon-sulfur polymer materials with their xe2x80x94Smxe2x80x94 groups, as described above, incorporated into the polymer backbone chain by covalent bonding of their terminal sulfur atoms; and carbon-sulfur polymer materials with greater than 75 weight per cent of sulfur in the carbon-sulfur polymer material.
In one embodiment, the anode of the cell comprises one or more materials selected from the group consisting of: lithium metal, lithium-aluminum alloys, lithium-tin alloys, lithium-intercalated carbons, and lithium-intercalated graphites.
Yet another aspect of the present invention pertains to a method of forming an electric current producing cell, the method comprising the steps of: (i) providing a cathode; (ii) providing an anode; and, (iii) interposing a nonaqueous electrolyte between the anode and the cathode, said electrolyte having a viscosity and an internal resistivity, the electrolyte comprising: (a) one or more solvents; (b) one or more ionic salts; and, (c) a multifunctional monomer comprising two or more unsaturated aliphatic reactive moieties per molecule, which multifuntional monomer is soluble in the one or more solvents, which multifunctional monomer rapidly polymerizes when the electrolyte is heated to an initiation temperature greater than 100xc2x0 C., thereby increasing the viscosity and internal resistivity of the electrolyte.
Still another aspect of the present invention pertains to a method for increasing the safety of an electric current producing cell comprising the steps of (i) providing a nonaqueous electrolyte, the electrolyte having a viscosity and an internal resistivity, the electrolyte comprising: (a) one or more solvents; (b) one or more ionic salts; and, (c) a multifunctional monomer comprising two or more unsaturated aliphatic reactive moieties per molecule, which multifuntional monomer is soluble in the one or more solvents, which multifunctional monomer rapidly polymerizes when the electrolyte is heated to an initiation temperature greater than 100xc2x0 C., thereby increasing the viscosity and internal resistivity of the electrolyte; and, (ii) incorporating said electrolyte into an electric current producing cell by interposing said electrolyte between a cathode and an anode. In one embodiment, upon rapid polymerization of the electrolyte at the initiation temperature, the viscosity and internal resistivity of the electrolyte increase to a level sufficient to safely terminate current producing operation of the cell.
Additional preferred embodiments are described below. As will be appreciated by one of skill in the art, features of one aspect or embodiment of the invention are also applicable to other aspects or embodiments of the invention.